Re: [PATCH bpf-next v5] bpf: add new helper get_file_path for mapping a file descriptor to a pathname

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Fri, Nov 1, 2019 at 5:57 AM Wenbo Zhang <ethercflow@gmail.com> wrote:
>
> When people want to identify which file system files are being opened,
> read, and written to, they can use this helper with file descriptor as
> input to achieve this goal. Other pseudo filesystems are also supported.
>
> This requirement is mainly discussed here:
>
>   https://github.com/iovisor/bcc/issues/237
>
> v4->v5: addressed Andrii and Daniel's feedback
> - rename bpf_fd2path to bpf_get_file_path to be consistent with other
> helper's names
> - when fdget_raw fails, set ret to -EBADF instead of -EINVAL
> - remove fdput from fdget_raw's error path
> - use IS_ERR instead of IS_ERR_OR_NULL as d_path ether returns a pointer
> into the buffer or an error code if the path was too long
> - modify the normal path's return value to return copied string lengh
> including NUL
> - update this helper description's Return bits.
>
> v3->v4: addressed Daniel's feedback
> - fix missing fdput()
> - move fd2path from kernel/bpf/trace.c to kernel/trace/bpf_trace.c
> - move fd2path's test code to another patch
> - add comment to explain why use fdget_raw instead of fdget
>
> v2->v3: addressed Yonghong's feedback
> - remove unnecessary LOCKDOWN_BPF_READ
> - refactor error handling section for enhanced readability
> - provide a test case in tools/testing/selftests/bpf
>
> v1->v2: addressed Daniel's feedback
> - fix backward compatibility
> - add this helper description
> - fix signed-off name
> ---

See nit below, but I'm fine with the current state as well.

Acked-by: Andrii Nakryiko <andriin@fb.com>

>  /* integer value in 'imm' field of BPF_CALL instruction selects which helper
>   * function eBPF program intends to call
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index f50bf19f7a05..fc9f577e65f5 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -683,6 +683,53 @@ static const struct bpf_func_proto bpf_send_signal_proto = {
>         .arg1_type      = ARG_ANYTHING,
>  };
>
> +BPF_CALL_3(bpf_get_file_path, char *, dst, u32, size, int, fd)
> +{
> +       struct fd f;
> +       char *p;
> +       int ret = -EBADF;
> +
> +       /* Use fdget_raw instead of fdget to support O_PATH, and
> +        * fdget_raw doesn't have any sleepable code, so it's ok
> +        * to be here.
> +        */
> +       f = fdget_raw(fd);
> +       if (!f.file)
> +               goto error;
> +
> +       /* d_path doesn't have any sleepable code, so it's ok to
> +        * be here. But it uses the current macro to get fs_struct
> +        * (current->fs). So this helper shouldn't be called in
> +        * interrupt context.
> +        */
> +       p = d_path(&f.file->f_path, dst, size);
> +       if (IS_ERR(p)) {
> +               ret = PTR_ERR(p);
> +               fdput(f);
> +               goto error;
> +       }
> +
> +       ret = strlen(p);
> +       memmove(dst, p, ret);
> +       dst[ret++] = '\0';
> +       fdput(f);
> +       goto end;
> +
> +error:
> +       memset(dst, '0', size);
> +end:
> +       return ret;

nit: I'd avoid unnecessary goto end (and end label itself) by having
two explicit returns:

    return 0;
error:
    memset(...);
    return ret;

> +}
> +
> +static const struct bpf_func_proto bpf_get_file_path_proto = {
> +       .func       = bpf_get_file_path,
> +       .gpl_only   = true,
> +       .ret_type   = RET_INTEGER,
> +       .arg1_type  = ARG_PTR_TO_UNINIT_MEM,
> +       .arg2_type  = ARG_CONST_SIZE,
> +       .arg3_type  = ARG_ANYTHING,
> +};
> +
>  static const struct bpf_func_proto *
>  tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>  {
> @@ -735,6 +782,8 @@ tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>  #endif
>         case BPF_FUNC_send_signal:
>                 return &bpf_send_signal_proto;
> +       case BPF_FUNC_get_file_path:
> +               return &bpf_get_file_path_proto;

This seems like a rather useful helper not just in tracing context. So
if maintainers are ok with that, maybe you can follow up with patch
that adds it in more BPF program types.

>         default:
>                 return NULL;
>         }

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